1. Field of the Invention
This invention covers improved electronic time-temperature indicators with a visual output, improved thermal modeling algorithms, and other devices and methods by which the thermal history of a material may be monitored, and the subsequent fitness for use of the tracked material may be quickly ascertained by visual or other means. The invention is well suited to monitoring the thermal stability of complex materials that are not well characterized by the standard Arrhenius exponential degradation curve.
2. Description of the Related Art
Many materials in use in commerce, medicine, and other areas are perishable. That is, the materials have a tendency to deteriorate with time, and this tendency to deteriorate is often accelerated by exposure to higher temperatures. This tendency to deteriorate is often designated as a material's “stability”. A material that deteriorates slowly in response to higher temperatures over long periods of time is said to have a “high stability”. By contrast, a material that deteriorates quickly in response to higher temperatures is said to have a “low stability”. For simple materials, thermal degradation processes are usually well characterized by the well-known Arrhenius equation:
                    k        =                  C          ⁢                                          ⁢                      ⅇ                          (                                                -                  E                                RT                            )                                                          (        1        )            
Here k is the rate of deterioration, C is a constant, E is the activation energy of the reaction, R is the universal gas constant, and T is the temperature in degrees Kelvin.
For more complex materials, however, the simple Arrhenius equation is often not sufficient. Complex materials can be composed of many different molecular entities, each with different activation energies and possibly different phase transition temperatures. As a result, the thermal degradation curve for more complex materials can often be a relatively complex function, which may have inflection points, sharp transitions, and other significant deviations from Arrhenius equation (1).
Examples of deterioration includes spoilage in the case of biological materials, loss of potency in the case of drugs, loss of chemical reactivity in the case of chemicals, or alternatively formation of unwanted contaminants. Excessive deterioration eventually results in the material in question being rendered unfit to use, or even rendered dangerous. Thus for commerce, medicine, and other areas, the rapid detection of materials rendered unfit to use by an unacceptable thermal history is very important.
Additionally, there are alternative situations where a material must undergo a certain minimal thermal history before it becomes fit for use. There are many materials, and material treatment processes, commonly used for construction, manufacturing, food preparation, and pharmaceutical preparation, such as concrete setting, epoxy hardening, biological fermentation, cooking, pasteurization, sterilization and the like, where the material needs to be properly cured, incubated, or heat treated before the material is fit to use. Since curing, incubation, or heat treatment processes are often temperature dependent, typically taking longer to proceed at lower temperatures, such materials must undergo a certain minimal time-temperature history before they are fit for use.
As a result, visual time-temperature indicators (TTI) are widely used in many areas of commerce. These are typically small devices that are affixed to a container of thermally sensitive material. The TTI shares the same thermal history as the material, and gives the user a visual warning if the material has had an improper thermal history.
Visual time-temperature indicators are often used to verify that a perishable, temperature sensitive, product has been transported from the manufacturer to the user via a transport process that has preserved the “cold chain”. Here, a “cold chain” means a continuous system for conserving and preserving materials at precise refrigerated temperatures from production to use, so that the integrity of the materials is assured.
There are several different types of visual time-temperature indicator in present-day use. These are chemically based, and follow the simple exponential Arrhenius decay equation. As previously discussed, however, one drawback of such Arrhenius decay type indicators, is that not all materials follow simple Arrhenius decay kinetics throughout all temperature ranges of interest. As a result, prior-art TTI cannot adequately monitor all materials.
FIG. 1 shows a graph of the stability of a material with a simple Arrhenius decay curve (1), and a material with a more complex decay curve (2). Here the curved exponential Arrhenius decay equation has been linearized by plotting 1/(Temperature) in degrees Kelvin on the “X” axis, versus the logarithm of the material's lifetime (in hours) at various temperatures on the “Y” axis. Note that although material (1) can be successfully monitored with a simple (i.e. linear function in 1/Temperature vs. log lifetime plots) Arrhenius-curve TTI (3); material (2) requires a more sophisticated TTI (4) capable of accurately reproducing more complex (i.e. non-linear function in 1/Temperature vs. log lifetime plots) thermal degradation curves. Prior to the present invention, however, no such sophisticated visual TTI devices (4) existed.
For the purposes of this discussion, “simple” temperature functions (or stability curves) are defined as exponential Arrhenius curves that produce a line with a single defined slope and intercept when the temperature function is plotted on 1/(Temperature ° K) versus log (lifetime) plots; and “complex” temperature functions (or stability curves) are defined as functions that produce curves, or higher order shapes when the temperature function is plotted on 1/(Temperature ° K) versus log (lifetime) plots, such that a single slope and intercept is inadequate to describe the resulting plot.
There are several different brands of visual Arrhenius-type time-temperature indicators in current use. For example, TempTime Corporation, Morris Plains, N.J., produces the Heatmarker® Time-temperature indicator for medical use. This indicator, often used to insure the integrity of vaccines in third-world countries, relies upon the progressive darkening of a chemical indicator, normally placed in the center of a “bulls eye” visual colorimetric reference pattern. Upon initial production, the chemical indicator is light in color, and the center of the “bulls eye” is lighter than the surrounding area. However upon exposure to an excessive amount of temperature for an excessive amount of time, the center of the bull's eye becomes darker than the surrounding area. A user may thus quickly and easily assess the integrity of any material associated with the indicator by simply noting if the center of the bulls eye is lighter or darker than the surrounding colorimetric reference material.
The chemistry techniques underlying this methodology is disclosed by Baughman et. al. in U.S. Pat. No. 4,389,217, Prusik et. al. in U.S. Pat. No. 6,544,925; and in other patents.
An alternative chemically based visual time-temperature indicator is the MonitorMark™ indicator, produced by the 3M corporation, Saint Paul, Minn. The MonitorMark uses a wicking material, along with a colored indicator that slowly migrates up the wick at a rate that is dependent on time and temperature. The user may thus quickly ascertain how far up the wick the colored indicator has migrated, and quickly assess if the material associated with the time-temperature indicator is fit to use.
The chemistry techniques underlying this technology is disclosed by Arens et. al. in U.S. Pat. No. 5,667,303, and in subsequent patents.
There are other types of time-temperature indicator that do not produce a visible output, but rather require the use of instruments to interrogate the indicator, and determine the state of the indicator. For example, the Bioett Corporation, Sweden, produces a radio frequency identification (RFID) non-visual time-temperature indicator. This indicator combines a passive RFID unit with an Arrhenius type, enzyme based, degradable circuit component, such that as the indicator is exposed to excessive amounts of temperature for excessive amounts of time, the RFID signature of the tag changes. The techniques underlying this methodology are disclosed by Sjoholm et. al. in WIPO application WOO 125472A1.
Although this approach lends itself to very low cost time-temperature sensors, the lack of visual output is inconvenient for most users, who typically are not equipped with sophisticated RFID reading equipment. As a result, users without this specialized equipment will be unable to ascertain the status of the sensor. An additional drawback of Sjoholm et. al. is that the precise stability characteristics of this device are dependent upon tuning the specific degradation of a chemically based (enzymatic) Arrhenius type time-temperature sensor to match the degradation characteristics of an arbitrary product. This is a time-consuming and burdensome process that may not always result in a precise stability match between the characteristics of the chemical time-temperature indicator, and the characteristics of the monitored material.
In addition to time-temperature indicators, which integrate time and temperature, and then make some sort of internal judgment as to if the unit has exceeded some preset criteria, there are a number of time-temperature data logging devices on the market. These logging devices typically store a record of the temperature history of the logger, and make the detailed history available for download to the user. However data loggers of prior art do not attempt to interpret this detailed history. Thus for prior art data logger devices, the interpretation of the relatively long and complex table of time and temperature log entries generated by the logger usually requires downloading the data, followed by a relatively sophisticated analysis by the user. It is clear that such devices impose a considerable burden on unsophisticated users, who simply want to quickly know if the material associated with the device is appropriate for use or not.
One example of a prior art data logger device is the Dallas Semiconductor iButton Thermochron series of temperature logger products. This data logger consists of a roughly ¾ inch diameter metal button that contains an internal battery, thermocouple, microprocessor, and data storage means. The iButton takes up to one million temperature readings over a time period of up to ten years, and stores these readings in its internal memory. Users may access the data by making electrical contact with the iButton through its 1-Wire electrical interface, and downloading the data into a computerized reader. This data then may be manipulated as the user desires, and assessments of the degradation status of the associated product may subsequently be made after additional analysis.
The techniques underlying these methods are taught by Curry et. al. in U.S. Pat. No. 6,217,213.
Other data loggers are also on the market. These include the HOBO time-temperature data logger produced by Onset Computer Corporation, Pocasset, Mass., and others. As does the Thermochron product, these other data loggers also acquire data from temperature sensors, store the data and time in an onboard memory, and make the data available for download and subsequent analysis by sophisticated users.
Electromechanical data loggers are also on the market. For example, the Monitor In-transit temperature recorder, produced by Monitor Co, Modesto, Calif. uses a battery operated, quartz-controlled clock motor to move a small strip of chart recorder paper past a bimetallic, temperature responsive, scribe to produce a visual strip-chart containing a detailed record of the thermal profile of the unit.
Another type of device is the temperature alarm. An example of this later type of device is the TagAlert® monitor, produced by Sensitech Corporation, Beverly Mass. This is a small electronic device, with a microprocessor, temperature sensor, battery, and display all enclosed in a single case. The device can be factory customized to notify the user if the device has exceeded any one of 4 preset alarm conditions, such as temperature went too low, temperature went too high, total time spent at a predetermined first temperature is too long, and/or total time spent at a pre-determined second temperature is too long. The device may be customized to respond to this narrow set of temperature alarm values, and pre-determined temperature-time alarm values.
The technology behind the TagAlert monitor was originally disclosed by Berrian et. al., U.S. Pat. No. 5,313,848; and subsequently reexamined and reissued as U.S. Pat. No. Re. 36,200.
In the broadest form, the device of U.S. Pat. No. Re 36,200 is a system, with an enclosed temperature sensor, which generates a time series of temperature measurements, stores some of the past time and temperature measurements, and uses some of these stored temperature measurements to generate an output signal. U.S. Pat. No. Re 36,200 differs from prior art electronic digital thermometers which also perform time series signal processing, and which also have digital memories of past readings, such as those taught by U.S. Pat. No. 4,536,851, in that the temperature sensor of U.S. Pat. No. Re 36,200 is enclosed in a housing, rather than on the surface of the housing or outside of the housing. In this respect, U.S. Pat. No. Re 36,200 has some aspects in common with electronic digital temperature controllers for portable medical instrumentation.
More specifically, however, the device of U.S. Pat. No. Re 36,200 may be viewed as a limited type of integrating time-temperature indicator, in that this device uses a sensor (isolated and protected from the external environment by a housing that also contains the other circuit components) to generate the time integral of temperature outside of an acceptable range, or above or below a predetermined threshold temperature, and store or otherwise make use of this value for output purposes.
Although the device of U.S. Pat. No. Re 36,200 teaches displaying a visual output means, the system has a number of drawbacks. In particular, the method is generally incapable of realistically modeling (or simulating) material thermal stability profiles, and thus is prone to generate inaccurate results.
U.S. Pat. No. Re 36,200 teaches a device that is essentially programmed by four parameters (the upper and lower acceptable temperature, the upper acceptable time value, and the lower acceptable time value). This method is very simplistic, however. The method assumes, for example, that no thermal changes occur between the upper and lower acceptable range conditions. Additionally, the method assumes that beyond the acceptable range limits, (at least up until an optional set of instantaneous temperature “stop” limits), all degradation occurs at the same rate regardless of temperature. As will be discussed in more detail later on, most materials have much more complex thermal degradation profiles, and are poorly monitored by such simplistic approaches.
Because of this lack of proper thermal modeling, for the purposes of this patent, the art of U.S. Pat. No. RE 36,200 will be designated as a “thermal alarm”. This nomenclature is consistent with the unit's commercial designation (TagAlert®).
The prior art for time-temperature indicators thus may be separated into three main types. One type consists of visual indicators, which use chemical means to mimic the Arrhenius degradation characteristics of a material of interest. These visual indicators may be directly interrogated by unsophisticated users using no additional equipment, and impose no significant analytical burden on the recipient of the material of interest.
The second type consists of non-indicating electronic time-temperature monitors, and electronic data loggers. This second type also monitors the time and temperature by chemical or electronic means, but does not output the data in a manner that is readily accessible to unsophisticated users without additional equipment. Rather, this second class of electronic device requires specialized reading equipment, and may additionally require sophisticated data analysis on the part of the recipient of the material of interest.
The third type consists of electronic time-temperature alarms. This device, exemplified by the Sensitech TagAlert® monitor, does not attempt to integrate the progressive effects of time and temperature over all probable thermal histories, but rather simply informs the user in the event that a limited number (absolute low, time 1 at low 1 exceeded, time 2 at high 2 exceeded, absolute high) of predetermined time-temperature excursions have taken place. U.S. Pat. No. 6,320,512 teaches similar time-temperature alarm methods, using circuit methods similar to those taught by Texas Instruments (MSP430 family Software Users Guide, 1994, p 9–18 to 9–21; MSP430 Family, Metering Application Report, 1997, p 42–45) and others.
Such devices are useful for monitoring conditions during shipping, such as determining if shipment ice packs have melted, detecting if a shipping container has been exposed to temperatures over 50° C., or detecting other standard shipping faults, but are less useful for monitoring the individualized stability profiles of arbitrary materials.
Radio-frequency based time-temperature indicators of the prior art, such as the previously mentioned device of Sjoholm et. al. (WO0125472A1), which contain Arrhenius based chemical timers, have many of the same accuracy drawbacks as chemically based visual indicators.
As a result of deficiencies in prior art TTIs, the present practice is to be conservative. That is, chemical time-temperature indicators are usually set to degrade more quickly than the material of interest. Although this scenario will insure that the user does not inadvertently accept degraded material, it is inefficient. In many cases, material that is, in fact, still good may be inappropriately discarded due to poor time-temperature indicator accuracy. Of course, the alternative scenario, in which the chemical time-temperature indicator fails to adequately warn that the tracked material is degraded, is both unacceptable and potentially dangerous.
By contrast, electronic data loggers have a different set of problems. Although these devices collect a full set of accurate time-temperature data, which may be used to determine if a material is acceptable or not, the data is in a difficult to interpret form. As previously discussed, many or most material recipients are unsophisticated, and are unlikely to have the equipment or specialized knowledge in order to read an electronic device, or to interpret a complex chart-recorder graphical output. As a result, many unsophisticated users, receiving material. associated with an unreadable or hard-to-read electronic tag, are likely to ignore the tag altogether. As a result, users may inadvertently use material that has been degraded by an unacceptable thermal history.
The temperature alarms of the prior art, such as U.S. Pat. No. Re. 36,200, also are not ideal. These alarms can only be adjusted to trigger on a limited set of fixed unacceptable temperature for a fixed unacceptable time combinations. They are not well suited to accurately mimic the stability characteristics of arbitrarily selected materials. As a result, they have a tendency to either trigger too soon, or too late, which can result in either waste, or inadequate warning.